In order to use superconducting technologies to measure electrical waveforms produced by room temperature devices, or indeed to interface any low temperature electronic device to a room temperature electronic device, the interface mechanism must satisfy certain electrical, mechanical, and temperature constraints.
U.S. Pat. No. 4,498,046 ('046) describes an interface which includes a pass-through liquid-helium-tight vacuum seal consisting of a flange and two half-cylindrical fused quartz portions, unequal in length, which act as a pass-through plug from a liquid-helium filled cryostat to a vacuum chamber. The two fused quartz half-cylinder portions of the pass-through plug are arranged so that the longer portion, which has copper striplines patterned on its flat surface, extends sufficiently beyond its mating half-cylinder portion on both ends to provide two platforms at opposite ends of the plug. A low temperature semiconductor chip or device is mounted on one of these platforms and a room temperature chip or device is mounted on the other. In operation, the pass-through plug is inserted through the cryostat wall such that the low temperature chip is immersed in liquid helium in the cryostat and the room temperature chip is disposed, near a heating element, inside the vacuum chamber. This chamber must be evacuated in order to prevent frosting of water and other gases on the plug, and also to provide adequate insulation for the cryostat.
The U.S. Pat No. 4,498,046 has numerous problems which render it costly and impractical to use in commercial applications. The U.S. Pat. No. 4,498,046 apparatus is typical of similar devices developed in the field of low temperature electronics which utilize liquid helium for achieving the necessary cooling temperatures.
A second example of the prior art which has drawbacks is a product used in the field of optics to cool devices, known by the trademark Heli-Tran and made by Air Products and Chemicals, Allentown, Pa. The Heli-Tran comprises a vacuum enclosed mounting head for holding a sample to be cooled, and a multi-channel flexible transfer tube for connecting the mounting head to a dewar of liquid helium. The transfer tube is believed to comprise a forward helium flow capillary (from the dewar to the mounting head), a shield tube surrounding the forward helium flow capillary, and a separate return flow capillary for the shield fluid. When the dewar is pressurized, liquid helium flows through both the forward helium flow capillary and the shield tube into the mounting head. The helium in the capillary strikes the inside surface of a metal block closing off the end of the transfer tube, then enters a passage coaxially surrounding all the transfer tube elements, travels a short distance in the return direction, and exits through a helium exhaust port. The helium in the shield tube turns back before the metal block, enters the return flow capillary, and exits from a shield flow return port near the dewar. The sample holder is attached to the outside of the metal block, so that it can conduct heat from the sample to be cooled into the metal block, which is itself cooled by the helium in the forward flow capillary.
The primary drawbacks with the Heli-Tran system are that the mounting head is entirely enclosed in a vacuum shroud, rendering sample demounting difficult and cumbersome and that the literature teaches total immersion of the sample, e.g., a superconducting electronic circuit, contributing to inefficient liquid helium consumption. In U.S. Pat. No. 3,894,403, FIG. 5, such a system is shown cooling a liquid helium bath in which a superconducting magnet is totally immersed.
Co-pending patent application Ser. No. 796,842, filed on Nov. 11, 1985, which is related to the present invention, discusses the aforementioned constraints and attempts of the prior art to satisfy them. The invention described and claimed in the said co-pending patent application obviates many of the foregoing problems. The device described therein includes a cooling chamber which encloses part of a sample to be cooled and a transfer tube which directs a stream of a cooling fluid through the cooling chamber to impinge upon the sample therein. The cooling fluid contacts the part of the sample within the chamber before being exhausted directly to the open air. Thus, the cooling chamber is not required to be a vacuum chamber and the device limits the surface area immersed in the cooling fluid However, accurate alignment of the stream of cooling fluid to insure that it strikes the part of the sample to be cooled was time-consuming and increased the cost of manufacturing.